The atomiser is the hollow part of the device. You insert the cartridge at the open end and screw the battery into the other, threaded, end. You will see a ceramic pot and a steel arch (or coil). The battery provides power to heat the coil, which vaporises the liquid in the damp spongy part of the cartridge.
E liquid can be dripped onto a spent cartridge to refill it, or directly onto the atomiser (this produces more vapour, which is important to many of us). Dripping directly onto the atomiser is considered more risky as it can clog or destroy an atomiser, and even leak into the battery. Also, some manufacturers say that dripping e-liquid directly onto an atomiser voids the warranty.
Regards
Emp